Fabrication of eco-friendly graphene-based superhydrophobic coating on steel substrate and its corrosion resistance, chemical and mechanical stability

Superhydrophobic coatings were successfully fabricated on steel substrates using potentiostatic electrodeposition of Ni and Ni-graphene, Ni-G, coatings followed by immersion in an ethanolic solution of stearic acid, SA. Rice straw, an environmentally friendly biomass resource, was used to synthesize high-quality graphene. The Raman spectra proved the high quality of the produced graphene. The Fourier transform infrared spectroscopy, FTIR, results showed that the Ni coating grafted with stearic acid, Ni-SA, and the Ni-G composite grafted with stearic acid, Ni-G-SA, were successfully deposited on the steel substrate. The scanning electron microscope, SEM, results showed that the prepared superhydrophobic coatings exhibit micro-nano structures. The wettability results revealed that the values of contact angles, CAs, for Ni-SA and Ni-G-SA coatings are 155.7° and 161.4°, while the values of sliding angles, SAs, for both coatings are 4.0° and 1.0°, respectively. The corrosion resistance, chemical stability, and mechanical abrasion resistance of the Ni-G-SA coating were found to be greater than those of the Ni-SA coating.

Superhydrophobic coating preparation. The steel substrate was mechanically polished with emery paper of various grades before electrodeposition, beginning with coarse one (grade 300) and progressing to the finest in stages (800 grade). The substrate was then degreased in a soap solution for 10 min, then activated by immersion in 2.0 M H 2 SO 4 for one minute, then rinsed with distilled water and ethanol before being directly immersed in the electrodeposition bath. The electrodeposition parameters for the fabrication of Ni coating and Ni-graphene, Ni-G, coating on the steel substrate are depicted in Table 1. A platinum sheet with the same dimensions as the steel substrate was utilized as an anode and was separated with a 2.0 cm gap from the steel substrate, the cathode. The Ni and Ni-G coatings were rinsed with distilled water and then dried at room temperature for a day. The dry coated Ni and Ni-G coatings substrates were immersed in ethanolic solutions of 0.01 M stearic acid (SA) for 0.25 h and then dried at room temperature. The prepared Ni coating grafted by stearic acid, Ni-SA, Table 1. Bath compositions and operating conditions for electrodeposition of Ni and Ni-graphene coating on the steel substrate. Surface characterization. A scanning electron microscope, SEM (model JSM-200 IT, JEOL), was used to examine the surface topography of the generated superhydrophobic coatings. The surface chemical composition was analyzed using the Fourier transform infrared spectrophotometer (model: Bruker Tensor 37 FTIR). The reported spectra are in the 4000-400 cm −1 . X-ray diffraction investigation was performed with monochromatic Cu K radiation (= 0.154056 nm) using an X-ray diffractometer (Bruker D2 phaser). Raman spectra of graphene were obtained using spectrometer (Senttera-Broker) equipped with 532 nm wavelength laser. Water contact angle (CA) and sliding angle (SA) were estimated with 5 µL water droplets using an optical contact angle goniometer (Rame-hart CA instrument, model 190-F2). The CAs and SAs values presented are the averages of two measurements carried out at different substrate locations.
Mechanical abrasion. The scratch test was utilized to analyze the mechanical abrasion properties of the produced superhydrophobic coatings. The prepared superhydrophobic coating samples were placed on 800 mesh sandpaper, and 3.0 kPa pressure was applied to them. The prepared superhydrophobic sample was moved horizontally, and the CA and SA were measured for each 3.0 cm abrasion length. The reported mechanical abrasion resistance is the average of values taken on two different samples.
Chemical stability. A water droplet of different pH values (pH = 1-13) was placed on the prepared superhydrophobic coatings, and the CAs and SAs were determined for each pH 43 . Sulfuric acid and sodium hydroxide were used to control the water droplet pH. The reported CAs and SAs are the average of two tests performed on the sample's surface at different places.
Corrosion tests. The electrochemical measurements were performed with a three-electrode cell on an ACM frequency response analyzer (UK). A graphite rod and an Ag/AgCl electrode were served as the counter and reference electrodes, respectively. The bare steel and steel coated by superhydrophobic Ni-SA and Ni-G-SA coatings were used as working electrodes. An epoxy layer was applied to the working electrodes, leaving 1 cm 2 exposed to the testing solution. The working electrode was placed in a cell containing 0.5 M NaCl solution that was opened to the atmosphere at room temperature and left for 20 min before electrochemical measurements to reach the equilibrium potential. The frequency range of the electrochemical impedance spectroscopy (EIS) measurements was 0.1 ≤ f ≤ 1.0 × 10 4 with an applied potential signal amplitude of 10 mV around the equilibrium potential. The polarization measurements were conducted at a 30 mV/min scan rate using a potential range of ± 250 mV around the equilibrium potential. Experiments were double-checked to ensure that the measurements were accurate and the results were within 2% error.

Results and discussion
Raman spectra. Raman scattering is a powerful non-destructive technique that is highly useful in examining the ordered and disordered crystallographic structure 44 . Figure 1 depicts the Raman spectrum of graphene. The D peak at 1286 cm −1 is produced by the breathing mode of the sp 2 atoms, which is active in the presence of defects and impurities in graphene 45 , whereas the G peak at 1621 cm −1 is generated by the E 2g phonon of sp 2 hybridized carbon atoms. Graphene also has a high 2D peak, around 2612 cm −1 . The 2D peak, on the other hand, is well known to be the second-order of the D peak. The number of layers has a significant influence on the shape, position, and strength of this peak relative to the D band. Therefore, a sharp 2D peak proved that graphene was successfully synthesized 46 . SEM and wettability results. Surface morphology is an important factor to consider while studying superhydrophobic coatings. Figure 3a shows a SEM micrograph of steel grafted by Ni-SA coating, demonstrating that the deposited nickel possesses nano-sized circular-like particles. Some of the nano-sized particles aggregates to form larger particles. Figure 3b depicts a micrograph of steel grafted by Ni-G-SA coating; the figure illustrates that the deposited nickel coating has nano-sized circular-like particles that are smaller in size than Ni-SA coating. Obviously, graphene could serve as a nucleation site to improve the nucleation rate, so the Ni-G-SA coating has smaller nano-sized particles 51,52 . So, the Ni-G-SA has higher surface roughness and thus greater superhydrophobicity. The transparent flakes of graphene sheets can easily be seen. To determine the wettability behaviour of the prepared superhydrophobic coatings, CAs and sliding angles, SAs, were measured. The values of CAs for Ni-SA and Ni-G-SA coatings are 155.7° and 161.4°, while the values of SAs for both coatings are 4.0° and 1.0°, respectively. These results indicate that; the presence of graphene increases roughness and superhydrophobicity. The nano-micro structures can store air that can effectively hinder water from contacting the surface 53 .
XRD results. The composition and crystal orientation of steel coated with Ni-SA and Ni-G-SA superhydrophobic coatings were determined using the XRD technique. Figure 4 depicts the XRD patterns of these coatings. For Ni-SA coating, there are three diffraction peaks at 2θ values of 44.6°, 64.7°, and 82.4° are, corresponding to faced cubic centered, fcc, of NiO (JCPDS card no. #47-1049). The (200) has the highest intensity of the three peaks, indicating that it is the preferred crystal orientation, with higher periodicity than the other orientations 54 .
For Ni-G-SA coating, there are two diffraction peaks; the peak at 2θ values of 21.6° corresponds to graphene, while that at 44.5 corresponds to Ni 55 . The graphene peak is broad, showing that graphene has a small particle size. The NiO peaks are absent in the presence of graphene.
Chemical stability. Figure 5a,b depict the relationships between the CAs and SAs of water droplets on the prepared superhydrophobic surfaces and pH. The results indicate that Ni-SA coatings are superhydrophobic in the pH range of 3-11, whereas the Ni-G-SA coatings are superhydrophobic in the pH range of 1-13, where the CAs are frequently larger than 150°, and the SAs are less than 10°. As a result, nickel doping with graphene improves the chemical stability of the superhydrophobic coating in both basic and acidic conditions. Table 2 summarizes the results of recent literature works on the chemical stability of the superhydrophobic surfaces on the steel substrate and their comparison with that of the prepared superhydrophobic coating in this study. As seen in the table, the prepared superhydrophobic coated steel has chemical stability superior to several previously recorded values. www.nature.com/scientificreports/ Mechanical stability. In industrial applications, poor mechanical abrasion of manufactured superhydrophobic coatings is regarded as a major issue. Improving the abrasion resistance of superhydrophobic coatings has been identified as a critical aspect for their industrial applications 56 . Some superhydrophobic surfaces are even fragile to the finger contact 57 . Figure 6a,b demonstrate the relationships between contact and sliding angles of water droplets on prepared superhydrophobic coatings as a function of the abrasion length. The plots demonstrate that the CAs decreased, and the SAs increased as the abrasion length increased. The prepared superhydrophobic Ni-SA coating maintains its superhydrophobicity until an abrasion length of 150 mm. While the prepared superhydrophobic Ni-G-SA coating maintains its superhydrophobicity up to a 300 mm abrasion length. These results revealed that doping the prepared superhydrophobic Ni-SA coating with graphene producing Ni-G-SA significantly improves the mechanical stability. The enhanced mechanical resistance of steel coated with Ni-G-SA is related to the excellent tribological behaviors of graphene [58][59][60][61] . Figure 7 depicts the SEM micrographs of steel coated with Ni-SA and Ni-G-SA coatings after the abrasion test. The figure shows that the nano-sized circular-like particles were destroyed for the prepared coatings. Since the low surface energy and surface roughness are two critical requirements for superhydrophobic coating fabrication, so the destroying of the nano-sized circular-like particles roughness causes the manufactured coatings to lose their superhydrophobic properties. Table 3 summarizes the findings of recent investigations on the mechanical abrasion resistance of superhydrophobic surfaces on steel substrates and their comparison to the produced superhydrophobic coating in this work. The prepared superhydrophobic coating is significant on the industrial scale as its good abrasion resistance, ease of the production procedure, as well as the availability and low cost of the chosen components.  Thus, the cathodic process is controlled by the oxygen gas diffusion from the bulk to the electrode surface. Table 4 depicts the potentiodynamic polarization parameters of the bare steel and superhydrophobic coated steel, including corrosion current density, i corr. , corrosion potential, E corr. , and protection efficiency, % P. Equation (2) was used to determine the protection efficiency 62 .
where, i o. and i are the corrosion current density for bare steel and superhydrophobic coated steel. The i corr. value for steel coated with Ni-SA is lower than that of bare steel, which can be related to the superhydrophobic behaviour of the coated steel. The trapped air around the superhydrophobic coating microstructures can reduce the contact area between the steel and the solution, resulting in a higher reduction in the i corr 63 . The presence of graphene improves the superhydrophobicity of the prepared Ni-G-SA coating, resulting in a greater decrease in the contact area of steel and the medium. So, the protection efficiency of steel coated by Ni-G-SA is higher than that of Ni-SA.
Electrochemical impedance spectroscopy results. The Nyquist and Bode plots of bare steel and superhydrophobic coated steel in 0.5 M NaCl solution are depicted in Fig. 9a-c. Nyquist plots, Fig. 9a, demonstrate a depressed capacitive semicircle, followed by a diffusion tail at low frequency. The depressed capacitive semicircle of the Nyquist plots at high frequencies is attributed to the interfacial charge transfer reaction 64 . The diffusion tail at low frequency is attributed to the diffusion process. Steel coated by Ni-G-SA shows the highest capacitive semicircle. The superhydrophobic coated steel blocks the active corrosion sites and limits the diffusion of the corrosive species, such as Cl − and H 2 O, into the surface of steel metal.
According to Fig. 9b, the Bode plots for steel coated by Ni-G-SA in 0.5 M NaCl solution show the largest impedance magnitudes at the low frequency while the bare steel has the lowest value. This is attributed to the protective action of the prepared superhydrophobic coats on the steel substrate. The phase angle plot, Fig. 9c, shows two times constant at low and moderate frequencies for bare steel and coated steel surface. The time www.nature.com/scientificreports/ constant appearing at the low-frequency range was due to the protective superhydrophobic coating or the corrosion products in bare steel. The time constant appearing at the moderate or high frequency was attributed to the electrical double layer [65][66][67] . The equivalent circuit shown in Fig. 10 was used to fit the EIS experimental data, and the impedance parameters were estimated by the Zsimpwin software. The equivalent circuit includes; solution resistance, R s , charge transfer resistance, R ct , double-layer constant phase element, CPE dl , and Warburg element. W. Table 5 depicts the EIS parameters of bare steel and superhydrophobic coated steel. The protection efficiency was determined using Eq. (3) 62 :  www.nature.com/scientificreports/ R ct o and R ct are the charge transfer resistance for the bare steel and superhydrophobic coated steel. Table 5 shows the obtained impedance parameters. Obviously, each of Rct and %P increase in the following order, bare steel < steel + Ni-SA < steel + Ni-G-SA, and so increasing the corrosion resistance in the same order. Table 6 summarizes the results of recent literature studies on superhydrophobic coating corrosion resistance on the steel substrate and compares them to the corrosion resistance of the produced superhydrophobic coating in this investigation. The data in the table show that the prepared superhydrophobic coating has good corrosion resistance, so it has significant in the industrial sector.
The enhanced corrosion resistance, chemical, and mechanical stability of the Ni-G-SA layer are due to its higher superhydrophobicity, the refined crystalline strengthening mechanism due to the small grain size of the nanostructures of the Ni-G-SA coating, the inclusion of graphene in a Ni matrix can effectively prevent dislocation sliding in the Ni matrix, the high chemical and mechanical stability, chemical inertness, impermeability, and hydrophobicity of graphene 34,52,[68][69][70][71][72][73][74] . In Addition, it is established that graphene helps in preventing the oxidation of metal at the expense of its own oxidation.
Mechanism of anti-corrosion performance. The water molecules can be freely adsorbed to the bare steel surface. Chloride ions can also be adsorbed to the steel surface and form [FeClOH] − , causing severe corrosion of uncoated steel. As a result, water and Cl − ions can easily contact the metal surface and start the corrosion process 75 .
The steel coated with superhydrophobic films, on the other hand, has a nanostructure that is covered by adsorbed hydrophobic material. Air can readily be trapped in the valleys between the rough surface's peaks. As a result of trapped air's obstructive influence, aggressive ion species such as Cl − in the electrolyte or corrosive environment can hardly attack the underlying surface 18,75,76 . The air trapped on the superhydrophobic surface really works as a passivation barrier between the substrate and the corrosive environment. Furthermore, because the isoelectric point for superhydrophobic materials in neutral solutions was at pH 2-4, it was determined that the superhydrophobic surface in neutral solutions was negatively charged. The negative charge of a superhydrophobic surface resulted in a decrease in the concentration of Cl − anion in the vicinity of a solid surface, which increased corrosion resistance 18 . It is reported that graphene has a negative zeta potential value due to the presence of electronegative functional groups formed at the graphite lattice [77][78][79] . So, the enhanced corrosion resistance of the steel coated with the superhydrophobic Ni-G-SA coating is due to its higher negative surface charge, so it has a lower concentration of Cl − anion in the vicinity of a solid surface than the steel coated with Ni-SA coating.  3. The prepared superhydrophobic Ni-G-SA coating has a water contact angle of 161.4 o , while the Ni-SA coating has a water contact angle of 155.7°. The presence of graphene improves the roughness of the prepared coat and so produces higher superhydrophobicity. 4. The chemical stability test indicates that the Ni-SA coating retains superhydrophobicity in the pH range 3-11, while the Ni-G-SA coating retains superhydrophobicity in the pH range 1-13. 5. The mechanical abrasion test showed that the prepared superhydrophobic Ni-SA coating exhibits superhydrophobicity until abrasion length of 150 mm; however, Ni-G-SA coating exhibits superhydrophobicity until abrasion length of 300 mm. 6. The presence of graphene in the prepared superhydrophobic coating improves its chemical and mechanical stability. 7. The potentiodynamic polarization results show that the corrosion current density values for the bare steel, steel coated by Ni-SA and Ni-G-SA in 0.5 M NaCl solution equal 0.057 mA/cm 2 , 0.0056 mA/cm 2 , and 0.0029 mA/cm 2 , respectively. The coating of steel with a superhydrophobic coating greatly decreases the corrosion current density, so the corrosion rate is greatly diminished. So, the doping of the superhydrophobic Ni-SA coating with graphene greatly improves the corrosion resistance behaviour. The electrochemical impedance spectroscopy results confirm the potentiodynamic polarization results.  www.nature.com/scientificreports/

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.
Received: 22 January 2022; Accepted: 6 June 2022 Table 5. The impedance parameters for the bare steel and superhydrophobic coated steel in 0.5 M NaCl solution.